This invention relates to a method of and an apparatus for controlling fuel flow in a one spool type gas turbine with a heat exchanger, particularly during the start-up or acceleration of its operation.
In a known method of controlling fuel flow for a one spool type gas turbine or engine with a heat exchanger when the rotation of the engine is started or accelerated, a reference fuel flow is determined in proportion to the air pressure at the compressor outlet. According to the method, there is every possibility that the temperature of the engine or the turbine blades may become higher than the tolerable range, when the rotation of the engine is started or accelerated with a large amount of heat energy stored in the heat exchanger. This is caused by the heat energy stored in the heat exchanger which raises the air temperature at the inlet of the combustion chamber of the engine. The amount of the heat energy greatly depends upon the time which has elapsed after the engine is stopped, atmospheric conditions and the like. In such a fuel flow control method, overheating of the engine during its start-up or acceleration is prevented to some extent by selecting a proper one from a plurality of predetermined fuel flow patterns showing the relation between the air pressure at the compressor outlet and the reference fuel flow in respect to the amount of heat energy stored in the heat exchanger. In practice, however, it is difficult to control timing between selection of a proper fuel flow pattern and the time which has elapsed after the engine is stopped, and stepped change of the combustion temperature of the fuel-air mixture increases the complication of the control system.
There has been proposed another fuel flow control method in which the rotational speed (rpm) of the engine is a parameter in place of the air pressure at the compressor outlet. In this system, however, the decrease of engine efficiency caused by a poor compressor performance, the drop of pressure in the heat exchanger or the decrease of air flow due to air leakage cannot be easily compensated, resulting in a bad influence upon combustion of the mixture.
There has been also provided a method in which a fuel flow pattern is selected in such a manner that the maximum fuel flow is determined irrespective of the temperature of the combustion gas at the turbine inlet and when the temperature of the combustion gas becomes such a value as to overheat the engine during its start-up or acceleration, fuel supply is decreased to prevent overheating of the engine. The abrupt decrease of the fuel supply is accompanied with dilution of the fuel-air mixture, which causes incomplete combustion with production of a large amount of hydrocarbons (HC) and carbon monoxide (CO), flame out, engine stopping or the like. The method also causes an abrupt change of temperature which has a bad influence upon durability of the engine.
Thus, the engine operation has been under the influence of the amount of heat energy stored in the heat exchanger when the engine is restarted. This is a definite defect of an automobile engine in which prompt and precise restarting of operation is required.